Composite membranes (separation membranes) are widely applied to a variety of fields including gas separation membranes, water disposal separation membranes, ion separation membranes, secondary battery separation membranes and the like. Materials for composite membranes are changed according to the corresponding application field. However, permeability is an essential requirement for excellent separation membranes, regardless of the type of the application field.
Conventional separation membranes focus on carbon membranes produced by finally carbonizing a single membrane made of a polymer. In general, carbon membranes are fine porous carbon membranes obtained by carbonizing membrane-type polymer precursors at high temperatures and the carbon membranes thus obtained have high permeability and selectivity, and long to term stability, durability, chemical resistance and high-temperature stability, but, disadvantageously, have bad mechanical properties such as elasticity and tensile strength, and entail an increase in production costs caused by a production process conducted at a high temperature of 600 to 1,000° C. for a long time, and low processability resulting from difficulty in formation of thin films, which are obstacles to commercialization, and have a big problem of occurrence of membrane defects during production (Patent Document 1: Korean Patent Laid-open Publication No. 2011-0033111).
In addition, as carbon nanotube membranes are reported to have excellent gas permeability and selectivity, research on composite membranes in which carbon nanotubes are incorporated in a polymer matrix is actively underway. However, the trade-off between gas permeability and selectivity is not still solved satisfactorily (Non-patent Document 1: Sangil Kim et al., J. Membr. Sci. 294 (2007) 147-158).
Recently, graphene, which has a single layer having a two-dimensional plane structure, exhibits excellent mechanical strength, and thermal and chemical properties and can be produced into a thin film, has been highlighted and cases in which composite membranes are produced by transferring graphene to porous polymer supports are reported. However, these composite membranes have problems of low permeability to several gases due to densely laminated structure of two-dimensional particles and relatively long permeation channel formed thereby (Patent Document 2: US Patent Publication No. 2012-0255899).
Meanwhile, research on separation membranes in which graphene oxide is coated on a support begins to be conducted in the application field of separation membranes. Representative examples of such separation membranes include gas separation membranes, water disposal separation membranes and ion separation membranes. Such research aims to utilize porous polymer supports coated with graphene oxide in gas separation membranes, water disposal separation membranes, ion separation membranes and the like. When porous polymer supports coated with graphene oxide are used for separation membranes, selectivity and permeability of subject substances to be separated can be improved. Accordingly, research using graphene oxide is continuously underway. In particular, there is research on incorporation of functionalized graphene, such as graphene oxide, into a porous polymer support to improve permeation flux or selectivity to a certain gas mixture (Patent Document 3: Korean Patent Laid-open Publication No. 2013-0128686).
However, there is still a demand for further improved permeability of separation membranes, although the separation membranes are produced by coating supports with graphene oxide, like Patent Document 3.
In addition, composite membranes with a simple structure including laminated oxide graphene, for example, composite membranes produced by coating with graphene oxide, like Patent Document 3, are disadvantageously easily delaminated by adjacent physical and chemical stimulus.
In particular, when separation membranes are produced by coating supports with graphene oxide, like Patent Document 3, attractive force between graphene oxide and the porous polymer support may be lowered. In this case, the graphene oxide coating layer is disadvantageously easily delaminated due to weak bonding force between the porous polymer support and the graphene oxide coating layer. In addition, the biggest problem occurring when separation membranes are produced by coating supports with graphene oxide is deteriorated permeability due to dense structure of the porous polymer support. That is, there is a problem in which the effects of improving permeability and selectivity owing to the graphene oxide coating layer are reduced by low permeability of the porous polymer support.